Road bumps surface channel (Forza) uses the max rumble of any wheel instead of weighting by contact

[Mhytee]

Issue

In Forza, road bumps' surface channel renders from the game's per-tire
SurfaceRumble[] taken as the max of any wheel. Like #30, that is load-blind:
one wheel brushing gravel reads as the whole car on gravel, and a
free-spinning airborne wheel may spike the channel mid-air. A source comment
attributes FH6 jump reports partly to this, but that was never isolated and
could be a misattribution; the test settles it.

Test

In Forza Horizon 6: disable all effects except road bumps, disable airborne
ducking, jump.

• Before fix: continuous rumble while airborne confirms the mid-air spike;
  silence clears that part (the load-blind weighting is still worth fixing).
• After fix: silent while airborne; gravel/dirt texture and rumble strips
  still trigger road bumps haptics, in proportion to how many wheels are on
  the surface.

Possible fix

Weight each wheel's SurfaceRumble by that wheel's suspension droop before
combining, mirroring #30's load-weighted average: an airborne wheel
contributes nothing, one wheel off-surface reads as its share, all four
read full strength.

Related decision: when the surface channel is available, should heave drive
the effect at all? On rough surfaces the two channels stack (double-applying
texture). Whether heave adds anything unique depends on whether SurfaceRumble
also carries vertical impacts: kerb strikes demonstrably spike it,
landings/potholes are unverified. Worth checking during the FH6 test; if
SurfaceRumble covers impacts too, heave could be disabled entirely whenever
the surface channel is active.

Related: #30, #32, #33.

[Mhytee]

Fixed on the beta branch (v0.2.0), in testing. Forza's surface-rumble channel now weights each tyre's rumble by suspension load before combining, instead of taking the max, so an airborne/lifted wheel or one wheel brushing a rough edge no longer reads as the whole car on that surface. Same load-weighting as #30.